OK, I've been debugging with No$SNS and I do notice a lot of flickering, but less of it if I slow down the dissolve animation. Unfortunately, the only way to get rid of the flickering entirely is to bring it back to what it is normally. For all I can fathom, either there's some other variable I can modify that can fix the flickering issue while retaining the sped-up animation, or what this comes down to is simply too much work for the DMA.
What causes the animation to be so slow is that C1/FD27, which is called 64 times, calls the function at C1/02F2 eight times itself, and one of the things C1/02F2 does is wait for an interrupt signal from the GPU thread; this signal is sent from a function located at C1/18D2. That means that over the course of the dissolve animation, we receive a total of 512 interrupt signals! It's no wonder the animation is so slow.
Stepping through C1/18D2, I noticed that one of the functions called near the beginning activates DMA channel 4 to draw VRAM on the screen. I refuse to believe that there isn't a way to speed up the animation without causing problems.
We have EVEN MORE themes now, but some old ones are gone...